Imaging optical unit for EUV projection lithography

ABSTRACT

An imaging optical unit for EUV projection lithography serves to image an object field into an image field. Mirrors guide imaging light from the object field to the image field. An aperture stop is tilted by at least 1° in relation to a normal plane which is perpendicular to an optical axis. The aperture stop has a circular stop contour. In mutually perpendicular planes, a deviation of a numerical aperture NA x  measured in one plane from a numerical aperture NA y  measured in the other plane is less than 0.003, averaged over the field points of the image field. What emerges is an imaging optical unit, in which homogenization of an image-side numerical aperture is ensured so that an unchanging high structure resolution in the image plane is made possible, independently of an orientation of a plane of incidence of the imaging light in the image field.

The contents of German patent application DE 10 2014 223 811.0 isincorporated by reference. In the event of conflict, the presentspecification controls.

FIELD OF THE INVENTION

The invention relates to an imaging optical unit for EUV projectionlithography. Furthermore, the invention relates to an optical systemwith such an imaging optical unit, a projection exposure apparatus withsuch an optical system, a production method for a microstructured ornanostructured component using such a projection exposure apparatus anda microstructured or nanostructured component produced therewith, inparticular a semiconductor chip, for example a memory chip.

BACKGROUND

An imaging optical unit for EUV projection lithography is known from US2007/0 223 112 A1, U.S. Pat. No. 6,781,671, US 2010/0 149 509 A1, WO2012/137 699 A1, US 2012/0069314 A1, EP 1 768 172 B1, WO 2004/046 771 A1and U.S. Pat. No. 7,751,127. The configuration of a system stop for animaging optical unit is known from U.S. Pat. No. 6,445,510 B1.

SUMMARY

It is an object of the disclosed technology to ensure a homogenizationof an image-side numerical aperture using the imaging optical unit sothat an unchanging high structure resolution in the image plane is madepossible, independently of the field location and of an orientation ofan image-side plane of incidence of the imaging light.

In general, in a first aspect, the invention features an imaging opticalunit for EUV projection lithography for imaging an object field in anobject plane into an image field in an image plane, the imaging opticalunit including a plurality of mirrors for guiding imaging light from theobject field to the image field, and an aperture stop, which is tiltedby at least 1° relative to a normal plane on which is perpendicular toan optical axis. The aperture stop is configured with a circular stopcontour and is arranged in such a way that the following applies tomutually perpendicular planes: a deviation of a numerical apertureNA_(x) measured in one of these planes from a numerical aperture NA_(y)measured in the other one of these two planes is less than 0.003,averaged over the field points of the image field.

A tilt of the aperture stop in relation to the normal plane which isperpendicular to the optical axis constitutes a previously unused degreeof freedom for homogenizing the image-side numerical aperture. By meansof such a tilted aperture stop, it is possible, in particular, to setthe maximum illumination angle for an image point in the image fieldvirtually independently of the direction of incidence and the imagepoint position in the image field. The tilt renders it possible tovariably predetermine the maximum illumination angle in a variationplane perpendicular to the tilt axis of the aperture stop by way of therespective tilt angle, independently of a plane perpendicular theretocontaining the tilt axis. Hence, tilting the aperture stop relative tothe normal plane ensures a change in a width corresponding to theprojection of the diameter of the aperture stop in the normal plane inone of two dimensions spanning the aperture stop and hence a change inthe numerical aperture predetermined by way of the width in this plane.This degree of freedom of a tilt of the aperture stop relative to thenormal plane enables a homogenization of the numerical apertures, inparticular in the case of off-axis fields and/or in the case of e.g.axial fields with an aspect ratio differing from 1, and thereforeenables an equalization of a maximum angle of incidence, independentlyof the direction of incidence. Therefore, the illumination angle isdefined by way of two angle coordinates, namely one angle whichcharacterizes the illumination direction in the respectively presentplane of incidence and a second angle which characterizes an azimuthorientation of the image-side plane of incidence. The homogenization ofthe numerical apertures NA_(x), NA_(y) for the two mutuallyperpendicular planes, which are spanned by the z-axis parallel to theoptical axis and an x-axis and by the z-axis and a y-axis lying in ameridional plane, is explained in detail in EP 1 768 172 B1 as animaging parameter to be optimized. In order to characterize thishomogenization, i.e. the deviation between the numerical aperturesNA_(x) and NA_(y), use can be made of the image field-side numericalaperture. The aperture stop with the circular edge can be manufacturedin a cost-effective manner. A diameter of the circular aperture stop canhave a variable configuration, for example in the style of an irisdiaphragm. The diameter can be modifiable in a predetermined manner withthe aid of an appropriate drive. The tilt of the circular aperture stopin relation to the normal plane on the optical axis has been found to beparticularly suitable, for example for adapting the numerical apertureNA_(x) and NA_(y). The optical axis is a common axis of rotationalsymmetry of the mirrors of the imaging optical unit. This assumes aconfiguration of the mirrors with rotationally symmetric reflectionsurfaces or with substantially rotationally symmetric reflectionsurfaces, i.e. apart from nano-asphere corrections. Such a configurationof the mirrors is not mandatory for all mirrors of the imaging opticalunit. To the extent that mirrors with non-rotationally symmetricreflection surfaces are used in the imaging optical unit, the opticalaxis is understood to mean a reference axis which is defined by theremaining rotationally symmetric mirrors or by rotationally symmetricreference surfaces with the best fit to the non-rotationally symmetricreflection surfaces. In addition to homogenization of the numericalaperture, the tilt of the circular stop can also lead to an improvementof the telecentricity and/or ellipticity and/or trefoil imagingparameters. As a result of the homogenization, the deviation of thenumerical aperture NA_(x) from the numerical aperture NA_(y), averagedover the field points of the image field, can be less than 0.003, lessthan 0.001, less than 0.0005 or e.g. 0.0002. An even smaller deviationin the region of 0.0001 is also possible. The deviation can be 0.00001.

Strictly speaking, the homogenization of the numerical apertures NA_(x)and NA_(y) over all field points in an image field, explained above,only relates to planes in the x-direction or y-direction. In particular,such a homogenization is advantageous for imaging H-lines or V-lines,i.e. for lines of an object structure to be imaged which extend parallelto the x-axis or parallel to the y-axis. The homogenization can alsoapply to different planes of incidence with arbitrary azimuth angles,i.e. to planes of incidence which have a finite angle in relation to thexz-plane of incidence or in relation to the yz-plane of incidence. Sucha homogenization, even for planes of incidence tilted correspondingly inrelation to the xz-plane or yz-plane, can be implemented, in particular,if structures which extend in the direction of a corresponding azimuthangle or perpendicular thereto are imaged.

In some embodiments, a stop arrangement departs from the prescription ofan arrangement of the stop plane in a manner coinciding with crossingpoints of individual imaging rays assigned to fixed illuminationdirections of various field points. It was found that an arrangement ofthe tilted circular aperture stop departing from the arrangement in thecoma plane or the arrangement in a chief ray crossing plane can lead toa further improvement of imaging parameters.

A corresponding statement applies to a decentration of the arrangementof the aperture stop. In order to define the coma plane and the chiefray crossing plane, the coma rays used in this document are definedbelow. Here, a distinction is made between free coma rays and boundingcoma rays.

Here, free coma rays are the rays which are transferred with maximumaperture from an off-axis article point or object point in an articleplane or else object plane to an image point in the image plane by meansof the imaging optical unit, with the rays intersecting at the imagepoint also having a maximum aperture. Here, the maximum aperture of therays intersecting at the image point, the aperture NA, is predeterminedas nominal variable or setpoint value for the imaging optical unit,with, initially, no aperture stop limiting the setpoint value of thenumerical aperture NA. The free coma rays are established by virtue of atelecentric centroid ray profile being assumed for each image point forthe imaging beams of the image points in the image plane. Using theseconditions of numerical aperture NA and telecentricity, an imaging beamto the associated object point is established for each image point,proceeding from the image points in the image plane against theprojection direction of the imaging optical unit in the direction of theobject plane. The rays bounding this imaging beam are referred to asfree coma rays or, more simply, as coma rays below. If the coma rays arebounded by a stop—an aperture stop—the rays which just pass the edge ofthe stop are also referred to as bounding coma rays.

The free coma rays of different article points or image points intersectalong a three-dimensional intersection line, which virtually form acircle for paraxial image points, with all free coma rays of the imagepoints approximately extending through this circle. For off-axis imagepoints, a closed three-dimensional intersection line emerges forrespectively two image points, along which three-dimensionalintersection lines the free coma rays intersect, with this intersectionline usually varying with the considered image points. The imaging beamsof all image points of the image field provide a waist region of theimaging beam from the sum of the intersection lines. This waist regionis generally referred to as a pupil. A coma plane for two image pointsemerges as a plane with the best fit to the sum of the intersectionlines. In order to determine this plane with the best fit, i.e. in orderto determine the coma plane, the three-dimensional intersection line canbe averaged in such a way that the integral of the squared perpendiculardistances of the intersection line along this line on the coma plane isminimal. Alternatively, it is also possible to use the distance or themagnitude of the distance for minimization when determining the comaplane. If more than two image points are considered, the coma planeemerges as that plane in which the sum of all integrals of theaforementioned type becomes minimal over all intersection lines. Below,the coma plane is also referred to as pupil plane.

Below, a distinction is made between chief ray plane and chief raycrossing plane.

The chief ray plane emerges as a plane perpendicular to the opticalaxis, which extends through the intersection point of a chief ray of anoff-axis object point with the optical axis. For paraxial object points,the chief ray is virtually identical to the centroid ray of the imaginglight beam, which transfers the object point in an object plane into animage point in an image plane with the maximum aperture angle by meansof the imaging optical unit. Here, the chief ray of the imaging lightbeam can initially extend parallel to the optical axis as far as thefirst mirror. Likewise, the chief ray can also extend parallel to theoptical axis after the last mirror of the imaging optical unit. Foroff-axis object points, the chief ray can deviate from the centroid ray.Furthermore, different intersection points of the chief rays emanatingfrom these object points with the optical axis emerge for differentobject points. The chief ray plane is defined as the plane with the bestfit in relation to these intersection points that is vertical to theoptical axis. The chief ray plane is that plane perpendicular to theoptical axis for which the integral of the squared distance of a chiefray intersection point from the plane becomes minimal, wherein theintegral is to be formed over all intersection points of chief rays offield points of an object field with the optical axis. The chief rayplane is also referred to as paraxial pupil plane.

The centroid ray is that imaging ray in a light beam which transfers anobject point in the object plane into an image point in an image fieldplane by means of the imaging optical unit, which centroid ray extendsthrough the energy-weighted centre in a plane perpendicular to thedirection of propagation thereof, which centre emerges by integratingthe light intensities of the aforementioned light beam in this plane.

The chief ray crossing plane is a plane parallel to the coma plane. Thechief ray crossing plane in this case extends through the region inwhich all chief rays of the imaging beams assigned to the image pointsunder the aforementioned conditions of NA setpoint value and theimage-side telecentricity intersect, or in which the sum of theseimaging beams has the narrowest cross section thereof. That is to say,the chief ray crossing plane is tilted about the intersection point ofthe chief ray plane with the optical axis, determined like above byaveraging, in such a way that it extends parallel to the coma plane.

Both the free coma rays and the chief rays constitute an ideal case inthe following figures. There, boundary or free coma rays or centre raysor chief rays for beams which extends exactly in a telecentric mannerfor all field points are shown. Moreover, these rays extend in the imageplane with a predetermined numerical aperture of the imaging opticalunit.

An orientation of a tilt axis for the aperture stop relative to anobject displacement direction was found to be particularly suitable. Anobject displacement drive can be provided for displacing the objectthrough the object field along the object displacement direction.

Certain values of the tilt angles of the aperture stopwere found to beparticularly suitable for achieving the object of good homogenization ofthe numerical aperture. The tilt angle can be at least 2.5°, can be atleast 3° or else can be even larger. The tilt angle can be smaller andcan be 1.1°. The tilt angle can be 8.6°. The tilt angle can be 13°. Thetilt angle can lie in the range between 5° and 15°. The tilt angle canlie in the range between 1° and 3°.

Certain orientations of a tilt relative to the angle of the optical axisrelative to the chief ray of the central field point were found to besuitable for improving imaging parameters, depending on the design ofthe imaging optical unit. What emerged unexpectedly here is that in factboth orientations can bring about an improvement, depending on thedesign of the imaging optical unit.

A planar stop configuration can likewise be manufactured in acost-effective manner. Alternatively, the aperture stop can be designedin such a way that it follows the form of a pupil of the imaging opticalunit, which may deviate from the planar design, in three dimensions.

A configuration of at least one mirror of the imaging optical unit as afree-form surface increases the degrees of freedom for the imagingoptical unit. All mirrors of the imaging optical unit can be configuredas free-form surfaces. Examples of free-form surface designs which canbe used within projection optical unit designs are found in WO 2013/174686 A1 and the references cited there.

In some embodiments, a drive increases the adaptation possibilities forthe imaging optical unit. The image-side numerical aperture can bepredetermined exactly and with a sufficient number of degrees of freedomby way of the tilt drive and the optionally likewise present drive forsetting the stop diameter.

Further aspects of the technology relate to the improvement of imagingparameters of an imaging optical unit when instead of using a titledcircular stop, an elliptical stop is used, which is displaced along areference axis, which may be the optical axis, or perpendicular thereto,in particular parallel to an object displacement direction, for thepurposes of optimizing parameters. These further aspects can be combinedwith the features explained above.

Imaging optical units of the first aspect may be incorporated in anoptical system for a projection exposure apparatus, used in productionof microstructured or nanostructured components.

Exemplary embodiments are explained in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a projection exposure apparatus for EUVmicrolithography;

FIG. 2 shows a meridional section of an imaging optical unit of theprojection exposure apparatus for imaging an object field into an imagefield, wherein the imaging optical unit has a tilted circular stop;

FIG. 3 shows a sectional magnification of the detail III in FIG. 2 inthe region of the tilted circular stop;

FIG. 4 schematically shows an imaging light beam profile in the regionof a circular aperture stop for elucidating used parameters;

FIG. 5 shows a top view of an object field of the imaging optical unitaccording to FIG. 2, wherein, in an exemplary manner and by way ofexample, uncorrected numerical apertures of two imaging light beamswhich emanate from two spaced apart field points are indicated;

FIG. 6 shows a diagram of a zeroth order of a Fourier expansion of anumerical aperture plotted for different field points, established forselected field points of the field according to FIG. 5;

FIG. 7 shows, in an illustration similar to FIG. 6, telecentricdistributions over two main directions as next order of the Fourierexpansion of the numerical aperture;

FIG. 8 shows, in an illustration similar to FIGS. 6 and 7, anellipticity as next order of the Fourier expansion of the distributionof the numerical aperture;

FIG. 9 shows, in an illustration similar to FIGS. 6 to 8, a trefoil asnext order of the Fourier expansion of the numerical aperture;

FIGS. 10 to 13 show, in illustrations similar to FIGS. 6 to 9, thenumerical aperture, telecentricity, ellipticity and trefoil imagingparameters for an arrangement modified in comparison with the stoparrangement according to FIGS. 2 and 3, in which the tilted circularstop is displaced both in a y-direction perpendicular to an optical axisand in a z-direction parallel to an optical axis for optimizing imagingparameters;

FIG. 14 shows, in an illustration similar to FIG. 2, a furtherembodiment of the imaging optical unit;

FIG. 15 shows a sectional magnification of the detail XV in FIG. 14 inthe region of a tilted circular stop;

FIGS. 16 to 19 show, in illustrations similar to FIGS. 6 to 9, thecorresponding imaging parameters for selected field points of theimaging optical unit according to FIG. 14;

FIG. 20 shows, in an illustration similar to FIG. 2, a furtherembodiment of the imaging optical unit;

FIG. 21 shows a sectional magnification of the detail XXI in FIG. 20 inthe region of a tilted circular stop;

FIGS. 22 to 25 show, in illustrations similar to FIGS. 6 to 9, thecorresponding imaging parameters for selected field points of theimaging optical unit according to FIG. 20;

FIGS. 26 to 29 show, in illustrations similar to FIGS. 6 to 9, thecorresponding imaging parameters for further selected field points ofthe imaging optical unit according to FIG. 20;

FIGS. 30 to 33 show, in illustrations similar to FIGS. 6 to 9, thecorresponding imaging parameters for selected field points of theimaging optical unit according to FIG. 2, wherein use is made of anon-tilted, circular stop;

FIGS. 34 to 37 show, in illustrations similar to FIGS. 6 to 9, thecorresponding imaging parameters for selected field points of theimaging optical unit according to FIG. 2, wherein use is made of anelliptical stop instead of a tilted circular stop;

FIGS. 38 to 41 show, in illustrations similar to FIGS. 6 to 9, thecorresponding imaging parameters for selected field points of theimaging optical unit according to FIG. 2, wherein use is made of anelliptical stop, instead of a tilted circular stop, wherein theelliptical stop is displaced perpendicular to the optical axis in they-direction compared with the arrangement according to FIGS. 34 to 37;

FIGS. 42 to 45 show, in illustrations similar to FIGS. 6 to 9, theimaging parameters for selected field points of the imaging optical unitaccording to FIG. 14, wherein use is made of an elliptical stop which isdisplaced perpendicular to the optical axis in the y-direction;

FIGS. 46 to 49 show, in illustrations similar to FIGS. 6 to 9, theimaging parameters for selected field points of the imaging optical unitaccording to FIG. 20, wherein use is made of an elliptical stop; and

FIGS. 50 to 53 show, in illustrations similar to FIGS. 6 to 9, theimaging parameters for selected field points of the imaging optical unitaccording to FIG. 20, wherein use is made of an elliptical stop which isdisplaced perpendicular to the optical axis in the y-direction.

DETAILED DESCRIPTION

A projection exposure apparatus 1 for microlithography includes a lightsource 2 for illumination light or imaging light 3. The light source 2is an EUV light source, which generates light in a wavelength range ofe.g. between 5 nm and 30 nm, in particular between 5 nm and 15 nm. Inparticular, the light source 2 can be a light source with a wavelengthof 13.5 nm or a light source with a wavelength of 6.9 nm. Other EUVwavelengths are also possible. A beam path of the illumination light 3is depicted very schematically in FIG. 1.

An illumination optical unit 6 serves to guide the illumination light 3from the light source 2 to an object field 4 in an object plane 5. Usinga projection optical unit or imaging optical unit 7, the object field 4is imaged into an image field 8 in an image plane 9 with a predeterminedreduction scale. The projection optical unit 7 according to FIG. 2 forexample has a reduction by a factor of 4. Other reduction scales arealso possible, e.g. 5×, 6×, or 8×, or else reduction scales with amagnitude greater than 8× or with a magnitude smaller than 4×, e.g. 2×or 1×. In the projection optical unit 7, the image plane 9 is arrangedparallel to the object plane 5. A portion of a reflection mask 10, whichis also referred to as reticle, coinciding with the object field 4 isimaged in this case. The reticle is supported by a reticle holder 11depicted schematically in FIG. 1, which is displaceable by way of areticle displacement drive 12, which is likewise depicted in a schematicmanner.

The imaging by the projection optical unit 7 is implemented onto thesurface of a substrate 13 in the form of a wafer, which is supported bya wafer holder or a substrate holder 14. The wafer holder 14 isdisplaced by way of a wafer displacement drive 15, which is likewisedepicted schematically in FIG. 1. Between the reticle 10 and theprojection optical unit 7, FIG. 1 schematically depicts a beam 15 a ofthe illumination light 3 entering therebetween and, between theprojection optical unit 7 and the substrate 13, a beam 15 b of theillumination light 3 exiting from the projection optical unit 7. Theillumination light 3 imaged by the projection optical unit 7 is alsoreferred to as imaging light. The imaging rays 15 b at the edge, whichare incident on the image field 8, are imaging rays which ideally belongto the same absolute illumination angles of the illumination of theimage field 8. The maximum angle of the exiting beam 15 b in relation tothe optical axis is a polar angle Θ. Θ therefore is the angle ofincidence in a plane of incidence, which has an optical axis oA and acoma ray of the beam 15 b. Θ ideally does not depend on the azimuthangle about the optical axis oA, i.e. on the direction of the plane ofincidence. In reality, the angle of incidence Θ is a function of anazimuth angle in the image plane 9. The numerical aperture, and hencethe variable determining the resolution capability of the imagingoptical unit, for example of the projection lens according to FIG. 1,emerges as NA=n*sin Θ, which is a function of the azimuth angle in theimage plane 9, particularly when imaging off-axis image points or anoff-axis image field 8, i.e. NA[azimuth]=n*sin [Θ (azimuth)]. Here, n isthe refractive index of the medium through which the exiting beam 15 bpropagates.

By way of example, an image field-side numerical aperture of theprojection optical unit 7 in the embodiment according to FIG. 2 is 0.26.This is not reproduced true to scale in FIG. 1. The image field-sidenumerical aperture can for example lie in the range between 0.2 and 0.7,depending on the embodiment of the projection optical unit 7.

In order to facilitate the description of the projection exposureapparatus 1 and the projection optical unit 7, a Cartesianxyz-coordinate system is indicated in the drawing, from which system therespective positional relationship of the components illustrated in thefigures is evident. In FIG. 1, the x-direction runs perpendicular to theplane of the drawing into the latter. The y-direction runs toward theright and the z-direction runs downward.

The projection exposure apparatus 1 is of the scanner type. Both thereticle 10 and the substrate 13 are scanned in an object displacementdirection parallel to the y-direction by a synchronized displacement byway of the displacement drives 12 and 15 during the operation of theprojection exposure apparatus 1. A stepper type of the projectionexposure apparatus 1, in which a stepwise displacement of the reticle 10and of the substrate 13 in the y-direction takes place betweenindividual exposures of the substrate 13, is also possible.

Both the object plane 5 and the image plane 9 respectively extendparallel to the xy-plane.

FIG. 1 depicts very schematically part of a beam path of the imaginglight 3 between the object field 4 and the image field 8. What is shownis the beam path of a chief ray 16 of a central field point of theprojection optical unit 7 routed over reflections at four mirrors M1,M2, M3 and M4 of the projection optical unit 7. This beam profile ismerely indicated very schematically in FIG. 1 in order to sketch out theregion around an aperture stop 18 which is still to be explained below.In addition to the mirrors M1 to M4, the projection optical unit 7 canhave further mirrors and, for example, can have a total of six, eight orten mirrors M1, . . . . The location, size and tilt of the mirrors M1 toM4 are merely shown schematically in FIG. 1 for indicating the beam pathof the imaging light 3, which generally has a different detailed route.At least one of the mirrors M1, M2, . . . can have a reflection surfacethat is embodied as a free-form surface.

The chief ray 16 crosses the optical axis oA of the projection opticalunit 7 in the beam path between the mirrors M2 and M3, wherein, forexample, the mirror M1 is the first mirror of the projection lens 7 inthe beam path downstream of the reticle 10. The optical axis oAconstitutes a reference axis, on the basis of which a mathematicalrepresentation of the optically used surfaces of the mirrors M1, M2, . .. of the projection optical unit 7 is possible. In particular, thereference axis can be an axis of rotational symmetry of these opticalsurfaces. The reference axis can lie in a symmetry plane of the imagingoptical unit 7. A crossing point of the chief ray 16 with the opticalaxis oA lies in a pupil plane 17 of the projection optical unit 7 in thecase of paraxial imaging. In order to distinguish this pupil plane 17from the pupil plane defined at the outset by the coma plane, it isdenoted as paraxial pupil or paraxial pupil plane below. The opticalaxis oA is perpendicular to a normal plane, which may coincide with theparaxial pupil plane 17, which is why the reference sign 17 is also usedfor the normal plane below. The aperture stop 18 of the projectionoptical unit 7 is arranged in the region of this crossing point of theoptical axis oA with the beam path of the chief ray 16 between themirrors M2 and M3. The aperture stop 18 is configured as a planar stop.

This aperture stop 18 serves to predetermine a homogenized image-sidenumerical aperture NA such that the image-side numerical aperturesNA_(x) perpendicular to the plane of the drawing of FIG. 1 and NA_(y) inthe plane of the drawing of FIG. 1 are as equal as possible for allimage points, i.e. NA_(x)=NA_(y) should be satisfied to the bestpossible extent. It is possible to achieve homogenization of thenumerical aperture in the case of a typical numerical aperture NA of0.25, in which the values between NA_(x) and NA_(y) of this numericalaperture NA vary by no more than 0.0025. An image-side numericalaperture shown in the plane of the drawing of FIG. 1 corresponds to thevalue NA_(y), since the numerical aperture in the yz-plane is depictedhere. An image-side numerical aperture NA_(x) emerges in the xz-planeperpendicular thereto.

FIG. 2 elucidates an actual course of imaging rays in the imagingoptical unit 7 between the object field 4 and the image field 8, whichboth lie at a distance from the optical axis oA, i.e. constituteoff-axis fields. What is shown in FIG. 2 is the course of individualrays 24 of the imaging light 3, which belong to five field points spacedapart from one another along the y-axis, wherein respectively one chiefray 16 and coma rays 19, 20 emanate from each one of these field points.As mentioned above, the chief rays and (free) coma rays for the idealcase of a setpoint aperture NA=0.25 are depicted in this case, with thebeams forming an image point extending telecentrically in the imageplane 9.

The basic design and the optical design data of the imaging optical unit7 according to FIG. 2 are known from EP 1 768 172 B1, apart from theexact course of the imaging rays and, even more critically, theconfiguration and arrangement of the aperture stop 18. The optical axisoA of the imaging optical unit 7 is the axis of rotational symmetry ofthe description of the surface of mirror surfaces of the mirrors M1 toM6 of the imaging optical unit 7.

FIG. 3 shows details in respect of the precise arrangement of theaperture stop 18, which is arranged in a stop plane 25. Here, thesection III from FIG. 2 is depicted in a magnified manner. Thecalculation of the beam paths depicted in FIG. 2 and FIG. 3 (and alsofor the following figures which show the course of imaging beams) wasinitially carried out without a stop and with a constant numericalaperture NA_(x)=NA_(y)=0.25 for the depicted image points in orderinitially to depict the ray route of the beams without a stop in thecase of these aperture and telecentricity conditions. After insertingthe real stop 18, the latter modifies the beams extending through theimaging optical unit 7 in such a way that these intersect in the regionof the edge of the stop; this has not been depicted so as to provide abetter overview.

Unlike in the configuration according to EP 1 768 172 B1, the aperturestop 18 has a circular configuration with a diameter of 66.254 mm. Inrelation to the normal plane 17 on the optical axis oA, the stop plane25 is tilted by a tilt angle α₀, the magnitude of which in theembodiment according to FIG. 2 and FIG. 3 is approximately 13°. Thistilt is implemented about a tilt axis 26. The tilt axis 26 isperpendicular to the yz-plane, i.e. perpendicular to a tilt normal plane27, which contains the object displacement direction y and in relationto which the field planes, i.e. the object plane 5 and the image plane9, are perpendicular. The tilt normal plane 27 coincides with themeridional plane of the imaging optical unit 7. In principle, othermagnitudes of the tilt angle α₀ in the region of between 10° and 16° arealso possible and can lead to an imaging performance of the projectionoptical unit that is improved over EP 1 768 172 B1, as will still beexplained in conjunction with the example in FIGS. 6 to 9.

This tilt about the tilt axis 26 through the tilt angle α₀ is broughtabout in an anticlockwise direction in the orientation of FIG. 3. In theembodiment according to FIGS. 2 and 3, the tilt of the circular aperturestop 18 is such that an angle co of a stop normal N_(AB) in relation toa chief ray 16 _(z) of a central field point is reduced in comparisonwith the angle γ₀ of the optical axis oA in relation to this chief ray16 _(z) of the central field point. In this case, α₀<0. ε₀=γ₀+α₀applies. The angle γ₀ simultaneously is the angle between a normal planes (cf. the subsequent FIG. 4) in relation to the chief ray 16 and thestop plane 25. Within this meaning, the circular aperture stop 18 isthus tilted relative to the overall beam of the imaging light 3 in sucha way that a projection of an area bounded by the aperture stop 18, asseen in the direction of the beam profile of this beam of the imaginglight 3, is enlarged compared to a positioning of the circular aperturestop 18 perpendicular to the optical axis oA.

For the purposes of a more detailed explanation of the tilt anglesituation for a tilted circular aperture stop, FIG. 4 shows, in aschematic manner and in extracts, a light beam 3 through an imagingoptical unit, e.g. a projection lens, which images an object point intoan image point, in a region in which a chief ray 16 intersects theoptical axis oA which extends along a z-axis. Depicted are an upper comaray 19, a lower coma ray 20 and the chief ray 16 of the light beam 3 ina yz-plane, which forms a meridional plane of the optical unit. Thelight beam 3 has a radius s in the depicted projection. The latteremerges from the distance between the upper coma ray 19 and theintersection point of the chief ray 16 with the optical axis oA, whereinthe distance s is measured perpendicular to the chief ray 16 and whereinthe chief ray 16 intersects the optical axis under the angle γ. A planeperpendicular to the optical axis oA through this intersection point ofchief ray 16 and optical axis oA is denoted as paraxial pupil plane 17,as described above. The paraxial pupil plane 17 intersects the uppercoma ray 19 in such a way that a distance r_(y) emerges in they-direction perpendicular to the optical axis oA, wherein this directione.g. corresponds to the object displacement direction in the case of aprojection lens. A stop perpendicular to the optical axis oA with thisradius r_(y) in the y-direction would bound the light beam 3 in thedepicted manner. The paraxial pupil plane 17 and a plane 17 aperpendicular to the chief ray through the intersection point betweenthe chief ray 16 and the optical axis oA are tilted against one anotherby the angle γ. In the case where the upper coma ray 19 and the chiefray 16 extend parallel to one another, the upper coma ray 19 is alsoperpendicular to the radius s. A radius s′ of the light beam 3 in thedirection of the lower coma ray 20 is generally different from theabove-described radius s in the direction of the upper coma ray 19.

If the light beam 3 has a radius r_(x) that differs from s in thex-direction (direction perpendicular to the plane of the drawing), whichradius equals the distance of the coma ray, not depicted in thisdirection, from the intersection point of the chief ray and the opticalaxis in the x-direction, wherein this distance is once again determinedperpendicular to the chief ray 16, the light beam cross section differsfrom the circular form.

If r_(x) is greater than s, the light beam depicted in FIG. 4 can bebounded by means of a circular planar stop with a radius r_(x) if thelatter is rotated or tilted about an axis of rotation 26 parallel to thex-axis through the intersection point between the chief ray 16 and theoptical axis oA, through an angle α in relation to the paraxial pupilplane 17, which extends through said intersection point. If r_(x) isgreater than r_(y) in this case, a is greater than zero. The circularstop 18 is therefore tilted in the anticlockwise direction compared withthe location of the stop perpendicular to the optical axis oA in FIG. 4.With the angle ε, introduced in the context of FIG. 3, for the angle ofthe stop normal in relation to the chief ray 16, which has an angle γrelative to the optical axis, ε=γ+α applies, with α>0. By contrast, ifr_(x) were smaller than r_(y), i.e. in a case not depicted in FIG. 4, ais less than zero. In this case, the relationship ε=γ+α also applies,with α<0. This case was explained above in conjunction with FIG. 3.

The following relationships: cos (γ)=s/r_(y); cos(γ+α)=s/r_(x) emergefrom FIG. 4, as result of which r_(y)/r_(x)=cos(γ+α)/cos(γ) emerges.Using this, it is possible to establish the tilt angle α in the case ofa predetermined diameter r_(x) of the circular stop and a predetermineddistance value r_(y).

It is mentioned that the light beam depicted in FIG. 4 cannot be boundedby a tilted circular planar stop in such a way that a setpoint value forthe numerical aperture in the x-direction and y-direction is to beapproximately equal, as averaged over all field point of the imagefield, for an imaging optical unit with a light beam 3 where r_(x) isless than s. However, in this case, the stop can be approximated by anelliptic stop, the semi-minor axis of which perpendicular to themeridional plane points in the direction of the axis of rotation whichextends through the intersection point of chief ray HS and optical axisoA. By way of example, this direction is perpendicular to the opticalaxis and perpendicular to the object displacement direction in the caseof a lithographic projection lens. The semi-major axis of the ellipticstop is determined by the tilt angle γ and the spacing of the coma rays19, 20 to be observed. This semi-major axis emerges, for example, byoptimizing the image-side telecentricity and the image-side numericalapertures NA_(x) and NA_(y), which should be as equal as possible forall image points.

It should be mentioned that the centre point of the tilted circular stop18, or, in accordance with a further aspect, of the elliptic tilted stop18 can be displaced from the above-described axis of rotation or tiltaxis by e.g. up to ±2 mm in the direction of the optical axis oA, i.e.in the z-direction. Using this it is possible, in particular, tooptimize an x-telecentricity, as will still be shown on the basis of thefollowing exemplary embodiments. Furthermore, a centre point or centreof these stops 18 in the y direction, i.e. in the object displacementdirection, can be displaced by e.g. ±1 mm. By way of this displacement,it is possible, in particular, to optimize a y-telecentricity, whichwill likewise still be shown below.

A mean numerical aperture of NA_(y) _(_) _(av)=0.24297 emerges for theprojection lens described in EP 1 768 172 B1 for a non-tilted, circularaperture stop with a diameter of r_(x)=66.254 mm, which is arrangedparallel to the object displacement direction, i.e. perpendicular to theoptical axis oA. The mean numerical aperture in the x-direction isNA_(x) _(_) _(av)=0.24952, and so a mean deviation of NA_(x) _(_)_(av)−NA_(y) _(_) _(av)=0.00655 emerges. A mean numerical aperture ofNA_(y) _(_) _(av)=0.25058 emerges when use is made of the elliptic stopdepicted in EP 1 768 172 B1, which has a semi-major axis in the objectdisplacement direction of r_(y)=68.348 mm and a semi-minor axisperpendicular to the optical axis and perpendicular to the objectdisplacement direction of r_(x)=66.254 mm, hence resulting in a ratio ofr_(y)/r_(x)=1.032. The mean numerical aperture in the x-direction inthis case is also NA_(x) _(_) _(av)=0.24952, and so a mean deviation ofNA_(x) _(_) _(av)−NA_(y) _(_) _(av)=0.00106 emerges. This is animprovement by approximately a factor of 6 compared to the firstmentioned, non-tilted circular stop. If the planar circular stop istilted by approximately −13°, as depicted in FIGS. 2 and 3, a meannumerical aperture of NA_(y) _(_) _(av)=0.24889 is obtained in the caseof a tilt angle of −12.9°. Here, the mean numerical aperture in thex-direction is NA_(x) _(_) _(av)=0.24984, and so a mean deviation ofNA_(x) _(_) _(av)−NA_(y) _(_) _(av)=0.00095 emerges, constituting animprovement in the mean deviation of the numerical aperture byapproximately a factor of 7 compared to the first mentioned, non-tiltedcircular stop. Here, the diameter of the tilted, planar and circularstop 18 was optimized to 66.366 mm. Furthermore, the centre Z of thestop was displaced in this case by the value of 1.216 mm along theoptical axis oA for optimizing the telecentricity. In addition to thisoptimization of the telecentricity, there additionally was adisplacement of the centre Z by 0.270 mm perpendicular to the opticalaxis oA in the y-direction. Using the optimized tilt angle of α=−12.9°,r_(y)/r_(x)=cos(γ+α)/cos(γ)=1.026 emerges. This example shows that themean deviations of the numerical aperture for the image points can befurther improved by means of the tilted and circular stop 18 with theplanar embodiment in view of the deviations when using an ellipticalstop described in EP 1 768 172 B1. It should also be particularlyhighlighted that the telecentricity for the lens described in EP 1 768172 B1 can be significantly improved by means of a tilted circular stop.The aforementioned data for the stop geometry and the numericalapertures were established over sixty-five field points distributed overthe image field 8.

As explained in the context of FIG. 4, the numerical aperture of theindividual image points in the image field 8 can be optimized by tiltingthe planar aperture stop 18 about an axis perpendicular to the objectdisplacement direction and perpendicular to the optical axis oA. Here,the numerical aperture NA for an image point in general is a function ofthe azimuth angle at the relevant image point in the image plane, whichis why (as shown above) NA(azimuth)=n*sin [Θ(azimuth)] applies for animage point. By means of the tilt angle α, about which the planaraperture stop 18 is tilted relative to the paraxial pupil plane, it ispossible to optimize the numerical aperture for predetermined, butgenerally arbitrary azimuth angles. The above-described optimizationsuch that the image-side numerical apertures NA_(x) and NA_(y) should beas equal as possible for all image field points corresponds to anazimuth angle=0° for NA_(x) and an azimuth angle=90° for NA_(y). Such anoptimization results in a tilt angle α. Alternatively, the numericalaperture of all image points can also e.g. be optimized to azimuthangles which respectively correspond to the maximum NA_(azimuth,max) andminimum aperture NA_(azimuth,min) for the respective image point suchthat the image-side numerical apertures NA_(azimuth,max) andNA_(azimuth,min) should be as equal as possible for all image points. Atilt angle α₁ optimized thus generally differs from the tilt angle α, inwhich the image-side numerical apertures NA_(x) and NA_(y) are as equalas possible for all image points. Optimizations of the tilt angle of theplanar stop are also possible such that the image-side numericalapertures NA_(azimuth) should be as equal as possible for all imagepoints, wherein this should then apply to a predetermined, fixed azimuthangle of e.g. 45°. Such an optimization of the tilt angle of a planarstop is advantageous if, for example, structures which extend under 45°in relation to the object displacement direction are imaged by theimaging optical unit.

As mentioned, it is generally possible for the centre Z or the centrepoint of the stop to be displaced along the z-axis, i.e. along theoptical axis. Displacements perpendicular to the optical axis in they-direction (object displacement direction) are also possible, as wasdescribed in the example above of the tilted circular stop in aprojection lens according to EP 1 768 172 B1. These displacements servefor the further optimization of the imaging properties, in particularfor optimizing the telecentricity. For the further optimization,displacements of the stop centre point in the x-direction (perpendicularto the object displacement direction) are also possible.

Therefore, a centre Z of the aperture stop 18 from FIG. 3 can forexample be at a distance from the intersection point of the stop plane25 with the optical axis oA, i.e. it is, in particular, at a distancefrom the optical axis oA.

Depicted additionally in FIG. 3 is a coma plane 28, in which the comarays 19, 20 from spaced apart field points intersect. This coma plane 28is likewise perpendicular to the tilt-normal plane 27, i.e.perpendicular to the yz-plane. The stop plane 25 of the aperture stop 18is tilted by an angle δ₀≠0 in relation to this coma plane 28. In theshown exemplary embodiment, the coma plane 28 is approximatelyperpendicular to the chief ray 16 _(z) of the central field point.δ₀=γ₀−α₀ applies approximately. In FIG. 3, a chief ray crossing plane29, in which the chief rays 16 from spaced apart field points intersect,extends parallel to the coma plane 28. The stop plane 25 is also tiltedby approximately the angle δ₀ in relation to the chief ray crossingplane 29. The pupil plane of the projection optical unit 7 lies in theregion of the planes 28, 29.

The crossing points 28 a, 28 b of the coma rays 19, 20 on the one handand 29 a of the chief rays 16 on the other hand are at a distance fromthe aperture stop 18.

The aperture stop optionally has a functional connection to a tilt drive18 a, to which the aperture stop 18 is connected for the tilt about thetilt axis 26. In particular, a step-free tilt of the aperture stop 18 ispossible by way of the tilt drive 18 a. The tilt drive 18 a can beconnected to a sensor arrangement, not depicted in any more detail inthe drawing, for measuring the image-side numerical apertures NA_(x),NA_(y) or NA_(azimuth) (for one or more predetermined azimuth angles),wherein a tilt setpoint value can be calculated from the measurementresult in a regulation unit (which is likewise not depicted here) andthis setpoint value can be fed by way of an appropriate signalconnection to the tilt drive 18 a for regulated readjustment of a tiltactual value and hence of the value for the image-side numericalaperture, in particular NA_(y). Thus, the yz-plane constitutes avariation plane for the image-side numerical aperture. The scanningdirection y lies in this variation plane.

Imaging properties of the projection optical unit 7 with the circularaperture stop 18 arranged in the aperture plane 25 are discussed belowon the basis of FIGS. 5 to 9.

FIG. 5 schematically elucidates a procedure of a numerical evaluation ofan imaging parameterization used here. FIG. 5 depicts a top view of theimage field 8 (from FIG. 1) of the imaging optical unit 7 for an objectfield 4 (from FIG. 1), which may have an arcuate embodiment, assigned tothis image field. In the numerical evaluation, a total of thirty-fivefield points FP are considered on the image field 8. Here, field pointsFP are selected which are lined up in one half of the image field 8 in amanner spaced apart equidistantly along the y-direction in a total ofseven field point columns 30 ₁ to 30 ₇. The five field points FP whichare lined up along the field point column 30 ₃ are schematicallyarranged in FIG. 5. Here, the field points FP are numbered in sequence,starting with the field point FP in the column 30 ₁ with the smallesty-coordinate and finishing with the field point FP₃₅ in the column 30 ₇with the largest y-coordinate.

A Fourier expansion of a distribution of a numerical aperture NA foreach one of the field points FP of the image field 8 is calculated forthese thirty-five field points FP, taking into account the opticaldesign data of the imaging optical unit 7. Here, a calculation over thefield points FP covering half of the image field 8 is sufficient. Tothis end, individual rays 24 (see e.g. FIGS. 1 and 2) of the imaginglight 3 are targeted onto the stop boundary of the circular aperturestop 18 (or, in general, of a stop situated in the imaging optical unit)against the projection direction, proceeding from each one of thethirty-five image field points FP. Examples of such numerical aperturesNA_(i) (with iε{1, 2, 3, . . . 35}) not yet optimized in respect of thetilt angle are indicated in FIG. 5 for the respective upper-most imagefield points FP of the field point columns 30 ₁ and 30 ₇. Thesenumerical apertures NA_(i) can be understood as a boundary of sub-beamsof the imaging light 3, which just still pass through the aperture stop18 starting from these image field points (against the actual beamdirection of the imaging light 3) or these sub-beams describe thenumerical aperture NA_(i) emerging for an image point i due to the stop.For the purposes of optimizing a tilt location of the aperture stop 18,the stop boundary of the aperture stop 18 is scanned equidistantly overthe circumference thereof. Then the respective direction cosines kx andky in the associated image point in the image field 8 are determined foreach one of these individual rays 24. This value is then converted intopolar coordinates k and φ. The Fourier expansion then enables access tothe deviations of the resulting numerical aperture for each one of theimage field points FP from a numerical aperture constant over allillumination directions of the respective field point. FIG. 5 depicts,by way of NA₇(φ) and NA₃₅(φ), the distributions of the numericalaperture as a function of the azimuth angle φ for the image field pointsFP₇ and FP₃₅. Field points distributed over the whole image field 8 canbe used for calculating NA.

In general, the Fourier expansion can be written as:

${N\;{A(\varphi)}} = {{NA}_{0} + {\sum\limits_{L = 1}^{N}\;\lbrack {{a_{L}\cos\; L\;\varphi} + {b_{L}\sin\; L\;\varphi}} \rbrack}}$

Here, NA₀ is a constant contribution, φ is the azimuth angle in theimage field plane in relation to an image point and a_(L), b_(L) areexpansion coefficients of the Fourier expansion.

Leading expansion terms of this Fourier expansion can be written asNA(φ)=NA ₀ +a ₁ cos φ+b ₁ sin φ+a ₂ cos 2φ+b ₂ sin 2φ+a ₃ cos 3φ+b ₃ sin3φ+ . . . .

These leading terms are:

NA₀: constant contribution of the numerical aperture independent of theazimuth (effective NA);

a₁: telecentricity in the x-direction;

b₁: telecentricity in the y-direction;

√{square root over (a₂ ²+b₂ ²)}: ellipticity

√{square root over (a₃ ²+b₃ ²)}: trefoil

FIGS. 6 to 9 provide results of this Fourier expansion. Here, theexpansion terms of the Fourier expansion are depicted up to the 3^(rd)order for a circular stop, which is arranged in the projection opticalunit described in EP 1 768 172 B1. Here, this circular stop is tilted byan angle of −13°, as was described in conjunction with FIG. 3. The stopdiameter in this case is 66.254 mm. The crossing point, e.g. of thecentral chief ray 16 _(z) with the optical axis oA, therefore lies inthe stop plane 25.

FIG. 6 shows the respective first expansion term NA₀ for the thirty-fivefield points FP. What emerges is a sawtooth-like profile, which showsthat the first coefficient of the Fourier series NA₀ above substantiallydecreases from the inner image field edge to the outer image edge in thedirection of the object displacement (scanning direction).

For elucidation purposes, FIG. 6 indicates an assignment of the depictedNA₀ curve to individual field point columns 30 ₁, 30 ₄ and 30 ₇. Theeffective numerical aperture NA₀ only varies very little between thevalues of approximately 0.2484 and 0.2496 over all thirty-five fieldpoints FP. A mean value of the numerical aperture in the xz-plane,NA_(x,average), emerges as 0.24954. A corresponding mean value in theyz-plane, NA_(y,average), emerges as 0.24867. The difference betweenthese two mean values, which are averaged over all thirty-five fieldpoints, is 0.00087, i.e. it is less than 0.001.

FIG. 7 shows the variation of the x-telecentricity and y-telecentricitytelecentricity values for the field points FP₁ to FP₃₅. For thex-telecentricity telecentricity value, slightly decreasing values to avalue of approximately −2 mrad emerge over these thirty-five fieldpoints, starting from a value of 0. The y-telecentricity telecentricityvalues vary between values of −2 mrad and −5 mrad. The y-telecentricitytelecentricity values once again vary in a stepped manner, comparablewith the variation of the effective numerical aperture NA₀ according toFIG. 6. Here, the telecentricity specifies the angle of the centroid rayof the light beam, which extends through one of the image points FP₁ toFP₃₅, wherein the aforementioned circular stop, tilted by −13°, isarranged in the projection optical unit described in EP 1 768 172 B1when imaging the image points. Here, the x-telecentricity is thedirection cosine in the xz-plane and the y-telecentricity is thedirection cosine in the yz-plane, wherein x, y and z in this case relateto a local coordinate system, the origin of which is at the consideredimage point, where z extends parallel to the optical axis, y extendsparallel to the object displacement direction (scanning direction) and xextends perpendicular to the y-axis and z-axis. If x-telecentricity andy-telecentricity are zero for an image point, the centroid ray of theassociated light beam associated with this image point is perpendicularto the image field plane 8. If the x-telecentricity is zero, thecorresponding centroid ray extends in the yz-plane. Analogously, thecentroid ray extends along the xz-plane when the y-telecentricity iszero.

FIG. 8 shows the value of the ellipticity for the thirty-five imagefield points FP.

The ellipticity describes the deviation of the aperture angle of arespective beam forming the respective image point by way of theinserted stop, e.g. the tilted circular stop, from the value NA₀ in thedirection of the main axes of the ellipse centred at the respectiveimage point in an elliptic approximation. Here, FIG. 8 elucidates thevariation of the numerical aperture along these ellipse coordinates andtherefore differs from the numerical apertures in the x-direction andy-direction NA_(x), NA_(y) for the corresponding image point, which iswhy, also, the corresponding mean values of these numerical aperturesdiffer over the considered number of image points. Thus, in FIG. 8, themean value over the depicted 35 field points is approximately 0.5*10⁻³.Averaging the numerical aperture NA_(x) over the 35 image point yieldsNA_(x)=0.24952 and approximately NA_(y)=0.24867, from which a differenceof 0.87*10⁻³ emerges. In practical terms, that type of averaging foroptimizing the stop will be preferred which is best fitted to thestructures to be imaged. By way of example, if horizontal and/orvertical structures are imaged, i.e. structures which are oriented inthe direction of the x-axis and/or y-axis, an optimization of theaperture in view of NA_(y) and/or NA_(x) is to be preferred. Ifstructures that are arranged at an angle not equal to 0° or 90° inrelation to the x-axis and y-axis are imaged, an optimization accordingto FIG. 8 is to be preferred.

FIG. 9 shows the variation of a further expansion term of the Fourierexpansion of the numerical aperture, the so-called trefoil. This valuevaries between 0 and 1×10⁻⁴. The values for the ellipticity and thetrefoil also vary in a stepped manner as a function of the evaluatedfield points FP_(i). Unlike the step-shaped variation of the effectivenumerical aperture NA₀ according to FIG. 6 and of the y-telecentricitytelecentricity value according to FIG. 7, the values within a respectivefield point column do not decrease, but slightly increase instead.

FIGS. 10 to 13 depict the expansion terms of the aforementioned Fourierexpansion for a further exemplary embodiment of the tilted circular stop18, wherein the latter is displaced along the y-direction and along thez-direction, i.e. along the optical axis oA, in terms of the stop centrepoint Z thereof. The displacement in the y-direction or objectdisplacement direction relative to the optical axis is 0.270 mm in thiscase, the displacement along the optical axis is 1.216 mm. The tiltangle is −12.95° and the post-optimized diameter is 66.366 mm. As aresult of the shift in the y-direction, it is possible to see asignificant improvement in the y-telecentricity (FIG. 11). Furthermore,an improvement in the x-telecentricity is achieved by a displacementalong the optical axis (FIG. 11).

In one exemplary embodiment of the tilted circular stop 18, the latteris only displaced along the y-direction but not along the z-direction interms of the stop centre point Z thereof. The displacement in they-direction or object displacement direction relative to the opticalaxis is 0.551 mm in this case. This y-displacement can lie in the rangebetween 0.1 mm and 1.0 mm and, for example, can also be 0.2 mm, 0.25 mm,0.27 mm or 0.3 mm. As mentioned above, no displacement was carried outalong the optical axis. The tilt angle corresponds to that which wasexplained above in conjunction with FIGS. 10 to 13. A post-optimizedstop diameter of the tilted and y-decentred circular stop 18 cancorrespond to that which was explained above in conjunction with FIGS.10 to 13. Alternatively, the post-optimized diameter of the circular,y-decentred stop 18 can also have a different value, e.g. the value of66.456 mm. What is found compared to the expansion terms described abovein conjunction with FIGS. 6 to 9 is that the use of the “displacement inthe y-direction” degree of freedom leads to a significant improvement inthe y-telecentricity, the absolute magnitude of which at most reachesthe value of 1.1 mrad over all measured field points. The additional useof the “displacement in the z-direction” degree of freedom describedabove in conjunction with FIGS. 10 to 13 then still leads to animprovement in the x-telecentricity. In absolute terms, a displacementin the z-direction is specified, proceeding from a location of a stoparrangement plane at the original design data, as mentioned in therespectively cited publications.

Below, a further embodiment of an imaging optical unit 31 with thetilted, circular aperture stop 32, which can be used in the projectionexposure apparatus 1 instead of the projection optical unit 7, isexplained on the basis of FIGS. 14 to 19. Components and functionscorresponding to those that were already explained above with referenceto FIGS. 1 to 13 are provided with the same reference signs and are notdiscussed in detail again.

In the meridional section according to FIG. 14, the individual rays 24of three field points spaced apart from one another in the y-directionare depicted in the beam path of the imaging optical unit 31, with, onceagain, the chief rays 16 and the coma rays 19, 20 being shown for eachone of these three field points. Apart from a configuration andarrangement of the aperture stop 32, the imaging optical unit 31corresponds to the one according to FIG. 5a of US 2007/0 223 112 A1 withthe associated description.

FIG. 15 shows, in a sectional magnification, the arrangement of the onceagain circular aperture stop 32 of the imaging optical unit 31. Theaperture stop 32 has a diameter of 51.729 mm. A diameter of 51.851 mm isalso possible.

The aperture stop 32 is tilted by an angle α₀−1.3° about the tilt axis26 in relation to the normal plane 17 on the optical axis oA. This tiltin relation to the normal plane 17 is brought about in the clockwisedirection in the orientation according to FIGS. 14 and 15.

The aperture stop 32 is tilted in such a way that the angle 130 of thestop normal N_(AB) in relation to the chief ray 16 _(z) of the centralfield point is reduced compared to the angle γ₀ of the optical axis oAin relation to the chief ray 16 _(z) of the central field point.ε₀=γ₀+α₀ applies in the aperture stop 32.

The coma plane 28 and the chief ray crossing plane 29 parallel theretoare once again plotted in FIG. 15. The stop plane 25 is tilted inrelation to both planes.

Crossing points 28 a, 28 b of the coma rays 19, 20 on the one hand and29 a of the chief rays 16 on the other hand are distant from theaperture stop 32.

For the purposes of optimizing the y-telecentricity, a centre Z of theaperture stop 32 can be distant from the intersection point of the stopplane 25 with the optical axis oA, i.e., in particular, distant from theoptical axis oA. The ideal distance in the y-direction is approximately0.28 mm in this case for the exemplary embodiment according to FIG. 14.Furthermore, the centre of the stop can be displaced in the direction ofthe optical axis in relation to the chief ray plane such that there isalso minimization of the x-telecentricity. A displacement value can liein the region of 1.2 mm, but it can also, for example, be significantlylarger and be e.g. 2.5 mm.

FIGS. 16 to 19 show the dependence of the leading expansion terms of theFourier expansion of the numerical aperture for the thirty-five fieldpoints FP for the circular stop tilted by −1.3° according to theexemplary embodiment according to FIG. 14 in accordance with what wasalready explained above in conjunction with FIGS. 6 to 9. In FIGS. 16 to19, a tilted, circular stop, which is displaced in the y-direction andz-direction for optimizing the telecentricity, is used in the imagingoptical unit 31 according to FIG. 5a of US 2007/0 223 112 A1. Here, thedisplacement in the y-direction is 0.28 mm and the displacement in thez-direction is 1.21 mm. The tilt angle α=−1.3°.

The effective numerical aperture NA₀ according to FIG. 16 varies betweenvalues of 0.2498 and 0.2501. Averaged over all thirty-five field points,a mean numerical aperture NA_(x,average) of 0.25039 emerges in thexz-plane and a mean value NA_(y,average) of 0.25060 emerges in theyz-plane. The difference of these mean values yields 0.00021.

The x-telecentricity telecentricity value varies between the values of 0mrad and −2 mrad and decreases monotonically between the field pointsFP₁ and FP₃₅. The y-telecentricity telecentricity value varies betweenthe values of 1 mrad and −1 mrad. The ellipticity value varies betweenthe values of 0 and 3×10⁻⁴. The trefoil value varies between the valuesof 2×10⁻⁵ and 3×10⁻⁵. The variations of NA₀, of the telecentricity valueof the y-telecentricity, of the ellipticity and of the trefoil onceagain have sawtooth structures for the respective field point columns 30_(i).

In the exemplary embodiment depicted in FIG. 14, the ratior_(y)/r_(x)=1.004, said ratio having been described in conjunction withFIG. 4. For the ideal tilt angle of the circular aperture stop, theratio cos(γ+α)/cos(γ)=1.005, which has good correspondence with theratio r_(y)/r_(x)=1.004.

FIG. 20 is used below to explain a further embodiment of an imagingoptical unit 33 with the tilted, circular aperture stop 34, which can beused in the projection exposure apparatus 1 instead of the projectionoptical unit 7. Components and functions corresponding to those thatwere already explained above with reference to FIGS. 1 to 19 areprovided with the same reference signs and are not discussed in detailagain. In this embodiment, the stop centre is displaced neither in they-direction nor in the z-direction.

Apart from a configuration and arrangement of the aperture stop 34, theimaging optical unit 33 corresponds to the embodiment according to FIG.1 in US 2003/0 076 483 A1, which is also published as U.S. Pat. No.6,781,671. Compared to the meridional section in FIG. 1 of US 2003/0 076483 A1, the projection optical unit 33 according to FIG. 20 is depictedin a manner mirrored about an axis parallel to the xy-axis.

In the embodiment according to FIGS. 20 and 21, the aperture stop 34 isalso circular and has a diameter of 31.032 mm. A diameter of 31.059 mmis also possible.

The aperture stop 34 is tilted in relation to the normal plane 17 by atilt angle α₀ of +8.6°. This tilt is once again implemented in theanticlockwise direction about the tilt axis 26 in the illustrationaccording to FIG. 21. This tilt is such that the angle β₀ of the stopnormal N_(AB) in relation to the chief ray 16 _(z) of the central fieldpoint is increased compared to the angle γ₀ of the optical axis oA inrelation to the chief ray 16 _(z) of the central field point. That is tosay, ε₀=γ₀+α₀ applies again.

In the exemplary embodiment depicted in FIG. 20, the ratior_(y)/r_(x)=0.920, said ratio having been described in conjunction withFIG. 4. For the ideal tilt angle of the circular aperture stop, theratio cos(γ+α)/cos(γ)=0.929, which has good correspondence with theratio r_(y)/r_(x)=0.920.

FIG. 21 once again depicts the coma plane 28 and the chief ray crossingplane 29. The stop plane 25 is respectively tilted by the angle δ₀ inrelation to these planes.

The crossing point 28 a of the coma rays 19 lies in the region of thestop plane 25 and in the region of an inner boundary of the aperturestop 34. The crossing point 28 b of the coma rays 20 and the crossingpoint 29 a of the chief rays 16 are distant from the aperture stop 34.

A centre Z of the aperture stop 34 is distant from the intersectionpoint between the stop plane 25 and the optical axis oA.

FIGS. 22 to 25 in turn show numerical results in the Fourier expansionof the numerical aperture when using the aperture stop 34 according toFIGS. 20 and 21. Depicted here are the results of five field points,which respectively lie in the centre of one of the field point columns30 _(i), which were already explained above in conjunction with theexplanations according to FIGS. 2 to 19.

The first term of the Fourier expansion, i.e. the effective numericalaperture NA₀, varies between values of 0.248 and 0.251 when using theaperture stop 34. Averaged over the field points, a value NA_(x,average)of 0.25137 emerges in the xz-plane and a value NA_(y,average) of 0.24924emerges in the yz-plane. A difference between these two mean values is0.00213.

An x-telecentricity telecentricity value lies very close to the value of0 mrad when using the aperture stop 34. A y-telecentricitytelecentricity value lies in the region between −15 and −14 mrad. Anellipticity value lies in the range between 0 and 5×10⁻⁴. A trefoilvalue lies in the range between 2.75 and 2.95×10⁻⁴.

In FIGS. 26 to 29, a tilted, circular stop which is displaced in they-direction and z-direction for optimizing the telecentricity is used inthe imaging optical unit 33 according to FIG. 1 of US 2003/0 076 483 A1.Here, the shift is −0.410 mm in the y-direction and −0.916 mm in thez-direction. The tilt angle α=+8.09° and the stop diameter was optimizedto 30.946 mm. The tilt axis is arranged analogously to FIG. 21. Acomparison between FIGS. 26 to 29 and 22 to 25 shows that theellipticity and the telecentricity are slightly better in the case ofthe tilted and displaced circular stop than in the case of thenon-displaced tilted circular stop.

The first term of the Fourier expansion, i.e. the effective numericalaperture NA₀, varies between values of 0.249 and 0.252 when using anappropriately displaced and decentred stop 34. Averaged over all fieldpoints, a value NA_(x,average) of 0.24984 emerges in the xz-plane and avalue NA_(y,average) of 0.24989 emerges in the yz-plane. A differencebetween these two mean values is 0.00095.

Accordingly, an optimization of the tilted, circular aperture stop 34,in which the latter is only decentred in the y-direction but notdisplaced in the z-direction, is also possible. Here, the y-decenteringcan have the value of −0.839 mm.

In order to compare the expansion terms of the Fourier expansion abovein the case of the tilted circular stop from FIG. 6 to FIG. 9 with anon-tilted circular stop, FIGS. 30 to 33 depict the correspondingexpansion terms of a circular stop with a diameter of 66.254 mm, withthe stop being arranged centrally in relation to the optical axis in thechief ray plane. It is possible to see a significant reduction of NA₀ ofapproximately 0.002 in the case of the non-tilted stop. Furthermore, thesystem with the non-tilted, circular stop has a y-telecentricity that isapproximately 50% worse (see FIG. 31) than a system with a circular stopthat is tilted by approximately −13° (see FIG. 7). A comparison of FIG.32 and FIG. 8 yields that the ellipticity is likewise worse byapproximately a factor of 4. A similar result applies to the trefoilsdepicted in FIGS. 9 and 33. Compared to an elliptical stop arrangedcentred in relation to the optical axis, which will still be describedbelow in conjunction with FIGS. 34 to 37, it was also found that thenon-tilted, circular stop has worse Fourier coefficients. A correctioneffect of the tilt degree of freedom for the stop is a correction of theellipticity, as emerges, for example from the data comparison above.

FIGS. 34 to 37 depict the expansion terms of the Fourier expansion up tothe third order for an elliptical stop, as is arranged and used in theprojection optical unit described in EP 1 768 172 B1. Here, the stop hasa semi-major axis of 68.348 mm in the object displacement direction(y-direction). Furthermore, the stop is perpendicular to the opticalaxis. The semi-minor axis of the stop is 66.254 mm in a direction(x-direction) perpendicular to the object displacement direction(y-direction). A comparison with FIGS. 6 to 9 shows that thetelecentricity, in particular the y-telecentricity, has significantlysmaller values in the case of the circular stop tilted by −13° than inthe case of the elliptic stop arranged perpendicular to the optical axisin the paraxial pupil plane. Furthermore, the ellipticity issignificantly reduced in the case of the tilted, circular stop, and sofor example, a mean value of 0.5*10⁻³ emerges in the case of the tiltedstop, whereas the ellipticity in the case of the non-tilted ellipticstop is approximately 2*10⁻³ and therefore worse by approximately afactor of 4 than in the case of the tilted stop. There likewise is animprovement in the trefoil in the case of the tilted circular stop.

FIGS. 38 to 41 likewise again depict the expansion terms of the Fourierexpansion up to the third order for an elliptical stop, as used in theprojection optical unit described in EP 1 768 172 with the dimensionsspecified above, wherein, however, the stop is displaced byapproximately 0.7 mm in the object displacement direction (scanningdirection). Hence, the centre of the elliptical stop is spaced from theoptical axis in the y-direction by 0.7 mm. As emerges from thecomparison of the respective FIGS. 34 to 37 and 38 to 41, a significantimprovement in the y-telecentricity and in the trefoil can be achievedby small displacements of the stop centre in the direction of the objectdisplacement direction by approximately 1.5 mm, by 0.7 mm in the shownexample, wherein the expansion coefficients for the zeroth and secondorder remain substantially unchanged in such a stop. Furthermore, thex-telecentricity can be improved by an optionally additionallyimplemented displacement of the stop centre point along the opticalaxis, i.e. in the z-direction.

The following mean values for the numerical apertures NA_(x) and NA_(y)in the x-direction and y-direction emerge for four of the six shownembodiments of the imaging optical unit 3 according to FIG. 3 from EP 1768 172 B1, which are summarized in Table 1, with the graphical displayof the Fourier coefficients for the non-tilted, circular stop arrangedin the paraxial pupil plane being dispensed with.

TABLE 1 Comparison of NA characteristic values for various stopconfigurations in an imaging optical unit as per FIG. 3 of EP 1 768 172B1 Table 1 NA_(x) _(—) _(average) NA_(y) _(—) _(average) DifferenceTilted, circular, 0.24954 0.24867 0.00087 non-displaced stop Tilted,circular, 0.24984 0.24889 0.00095 displaced stop Elliptical and 0.249520.25058 0.00106 displaced stop Non-tilted, 0.24952 0.24297 0.00655circular stop

Table 1 shows that better results in respect of the variation of thenumerical aperture of the image field can be obtained with tilted,circular aperture stops than with stops arranged in the paraxial pupilplane. Furthermore, the telecentricity can be significantly improved.

In FIGS. 42 to 45, an elliptic stop arranged in the paraxial pupilplane, i.e. arranged perpendicular to the optical axis in the chief rayplane, is used in the imaging optical unit 31 according to FIG. 5a of US2007/0 223 112 A1. Here, the centre point of this stop is displaced fromthe optical axis by 0.6 mm in the y-direction in order to adapt they-telecentricity. The stop has an extent of 51.840 mm in the x-directionand 52.052 mm in the y-direction. In the case of such a non-tiltedelliptic stop, the y-telecentricity can be improved by y-decentering.

A comparison between FIGS. 42 to 45 and FIGS. 16 to 19 shows that theellipticity and the telecentricity is slightly better in the case of thetilted and displaced circular stop than in the case of the displacedelliptic stop.

The following mean values for the numerical apertures NA_(x) and NA_(y)in the x-direction and y-direction emerge for the various embodiments ofthe imaging optical unit 31 according to FIG. 5a from US 2007/0 223 112A1, which are summarized in Table 2, with the graphical display of theFourier coefficients for the non-tilted, circular stop arranged in theparaxial pupil plane being dispensed with.

TABLE 2 Comparison of NA characteristic values for various stopconfigurations in an imaging optical unit as per FIG. 5a from US 2007/0223 112 A1 Table 2 NA_(x) _(—) _(average) NA_(y) _(—) _(average)Difference Tilted, circular, 0.24988 0.25008 0.00020 non-displaced stopTilted, circular, 0.25039 0.25060 0.00021 displaced stop Elliptical and0.24988 0.25013 0.00025 displaced stop Non-tilted, 0.24988 0.249120.00076 circular stop

Table 2 shows that better results in respect of the variation of thenumerical aperture of the image field can be obtained with tilted,circular aperture stops than with stops arranged in the paraxial pupilplane.

In FIGS. 46 to 49, an elliptic stop which is not displaced in they-direction and z-direction is used in the imaging optical unit 33according to FIG. 1 of US 2003/0 076 483 A1. The aperture stop isarranged in the paraxial pupil plane, i.e. perpendicular to the opticalaxis in the chief ray plane. The stop has an extent of 31.032 mm in thex-direction and 28.548 mm in the y-direction.

In FIGS. 50 to 53, a displaced elliptic stop which is displaced in they-direction and z-direction for optimizing the telecentricity is used inthe imaging optical unit 33 according to FIG. 1 of US 2003/0 076 483 A1.The aperture stop is arranged in the paraxial pupil plane, i.e.perpendicular to the optical axis in the chief ray plane. Here, thecentre point of this stop is displaced from the optical axis by −0.7 mmin the y-direction in order to adapt the y-telecentricity. The stop hasan extent of 31.032 mm in the x-direction and 28.548 mm in they-direction.

The following mean values for the numerical apertures NA_(x) and NA_(y)in the x-direction and y-direction emerge for three of the four shownembodiments of the imaging optical unit 33 according to FIG. 1 from US2003/0 076 483 A1, which are summarized in Table 3. Furthermore, themean values for a non-tilted, circular stop in the paraxial pupil planeare specified, with the graphical display of the Fourier coefficientsfor the non-tilted, circular stop being dispensed with.

TABLE 3 Comparison of NA characteristic values for various stopconfigurations in an imaging optical unit as per FIG. 1from US 2003/0076 483 A1 Table 3 NA_(x) _(—) _(average) NA_(y) _(—) _(average)Difference Tilted, circular, 0.25137 0.24924 0.00213 non-displaced stopTilted, circular, 0.25182 0.24997 0.00185 displaced stop Elliptical0.25136 0.24871 0.00265 displaced stop , Non-tilted 0.25138 0.270520.01913 circular stop

Table 3 likewise shows that tilted, circular aperture stops can achievebetter results in respect of the variation of the numerical aperture ofthe image field than stops arranged in the paraxial pupil plane.

As the various exemplary embodiments of aperture stops in the threeprojection lenses for EUV lithography depicted above show, it ispossible to achieve a homogenization of the numerical aperture of theimage field of the lens using a tilted, circular stop, which is arrangedin the region of the pupil of the projection lens, by way of a tiltangle optimization and optionally by way of displacing the stop in they-direction and/or z-direction, which correspond to the objectdisplacement direction and the optical axis, respectively. Here, betterresults are achieved than with non-tilted stops arranged in the paraxialpupil plane. Furthermore, the telecentricity and trefoil canadditionally be improved.

The magnitude of the tilt angle α₀ of the tilted, circular stopgenerally lies in the range between 1° and 20°, preferably in the rangebetween 5° and 15°. The tilt angle α₀ can be just as large as an angleα_(CR) between the chief ray 16 of the central field point and theoptical axis oA.

The exemplary embodiments of the imaging optical units described aboveeach have mirrors M1, M2, . . . , which can be described by way of anasphere equation, i.e. which have rotationally symmetric reflectionsurfaces, which are used in parts, in relation to the optical axis oA.Alternatively, an imaging optical unit not depicted in a figure can beused with an aperture stop arranged with an appropriate tilt, whichoptical unit contains at least one mirror embodied as a free-formsurface. Examples of such free-form surfaces are described in thefollowing publications: WO 2014 000 970 A1, WO 2013/174 686 A1, US2013/0 088 701 A1, US 2013/0 070 227 A1, US 2012/0 274 917 A1, U.S. Pat.No. 8,018,650 B2, U.S. Pat. No. 8,810,903, US 2013/0 342 821 A1, U.S.Pat. No. 7,414,781 B2, U.S. Pat. No. 7,719,772 B2, US 2012/0 188 525 A1,U.S. Pat. No. 8,169,694 B2.

For the purposes of producing a microstructured or nanostructuredcomponent, the projection exposure apparatus 1 is used as follows:initially, the reflection mask 10 or the reticle and the substrate orthe wafer 13 are provided. Subsequently, a structure on the reticle 10is projected onto a light-sensitive layer of the wafer 13 with the aidof the projection exposure apparatus 1. Then, by developing thelight-sensitive layer, a microstructure or nanostructure is generated onthe wafer 13 and hence the microstructured component is generated.

A number of embodiments are described. Other embodiments are in theclaims.

What is claimed is:
 1. An imaging optical unit for EUV projectionlithography for imaging an object field in an object plane into an imagefield in an image plane, the imaging optical unit comprising: aplurality of mirrors for guiding imaging light from the object field tothe image field; an aperture stop, which is tilted by at least 1°relative to a normal plane which is perpendicular to an optical axis,wherein the aperture stop is configured with a circular stop contour,wherein the aperture stop is arranged in such a way that the followingapplies to mutually perpendicular planes: a deviation of a numericalaperture NAx measured in one of these planes from a numerical apertureNAy measured in the other one of these two planes is less than 0.003,averaged over the field points of the image field.
 2. The imagingoptical unit of claim 1, wherein the stop is arranged at a distancefrom, or tilted relative to, a plane, in which coma rays of the imaginglight from spaced apart field points intersect.
 3. The imaging opticalunit of claim 1, wherein the aperture stop is arranged at a distancefrom, or tilted relative to, a plane, in which chief rays of the imaginglight from spaced apart field points intersect.
 4. The imaging opticalunit of claim 1, wherein a centre of the aperture stop is at a distancefrom a reference axis of the imaging optical unit.
 5. The imagingoptical unit of claim 1, wherein the aperture stop is tilted about atilt axis which is perpendicular to a tilt normal plane, which containsan object displacement direction for an object arrangeable in the objectplane and with at least one field plane being perpendicular thereto. 6.The imaging optical unit of claim 1, wherein the tilt angle is less than20°.
 7. The imaging optical unit of claim 1, wherein the aperture stopis tilted in such a way that an angle of a stop normal relative to achief ray of a central field point becomes smaller in comparison with anangle of the optical axis relative to the chief ray of the central fieldpoint.
 8. The imaging optical unit of claim 1, wherein the aperture stopis tilted in such a way that an angle of a stop normal relative to achief ray of a central field point becomes larger in comparison with anangle of the optical axis relative to the chief ray of the central fieldpoint.
 9. The imaging optical unit of claim 1, wherein the aperture stopis configured as a planar stop.
 10. The imaging optical unit of claim 1,wherein at least one of the mirrors has a reflection surface embodied asa free-form surface.
 11. The imaging optical unit of claim 1, furthercomprising a tilt drive, to which the aperture stop is connected for thepurposes of tilting.
 12. An optical system, comprising: the imagingoptical unit of claim 1; and an illumination optical unit forilluminating the object field with illumination light or imaging light.13. A projection exposure apparatus, comprising: the optical system ofclaim 12; and a light source for generating EUV illumination light orimaging light; an object holder with an object displacement drive; and asubstrate holder for holding a wafer, arrangeable in the image field,with a wafer displacement drive.
 14. A method for producing a structuredcomponent, comprising: providing a reticle and a wafer; projecting astructure on the reticle onto a light-sensitive layer of the wafer withthe aid of the projection exposure apparatus of claim 13; and generatinga structure on the wafer.